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Posted to issues@ozone.apache.org by "Ethan Rose (Jira)" <ji...@apache.org> on 2020/07/17 19:01:00 UTC
[jira] [Comment Edited] (HDDS-3976) KeyValueBlockIterator#nextBlock
skips valid blocks
[ https://issues.apache.org/jira/browse/HDDS-3976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17160023#comment-17160023 ]
Ethan Rose edited comment on HDDS-3976 at 7/17/20, 7:00 PM:
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Implementing seekLast() correctly and efficiently with a filter requires seeking to the end of the RocksIterator and stepping back over mismatched keys using its prev() method. Because KeyValueBlockIterator uses a MetaStoreIterator as a wrapper over the RocksIterator, and MetaStoreIterator does not support prev(), this method cannot be implemented in the current state of the code. Options:
# Add support for prev() to MetaStoreIterator.
# Remove KeyValueBlockIterator#seekToLast since it is only used in tests.
Current implementation removed KeyValueBlockIterator#seekToLast. It was not being used, and the instances it would be needed are minimal since the iterator cannot go backwards from the last element anyways.
was (Author: erose):
Implementing seekLast() correctly and efficiently with a filter requires seeking to the end of the RocksIterator and stepping back over mismatched keys using its prev() method. Because KeyValueBlockIterator uses a MetaStoreIterator as a wrapper over the RocksIterator, and MetaStoreIterator does not support prev(), this method cannot be implemented in the current state of the code. Options:
# Add support for prev() to MetaStoreIterator.
# Remove KeyValueBlockIterator#seekToLast since it is only used in tests.
> KeyValueBlockIterator#nextBlock skips valid blocks
> --------------------------------------------------
>
> Key: HDDS-3976
> URL: https://issues.apache.org/jira/browse/HDDS-3976
> Project: Hadoop Distributed Data Store
> Issue Type: Bug
> Reporter: Ethan Rose
> Assignee: Ethan Rose
> Priority: Major
>
> HDDS-3854 fixed a bug in KeyValueBlockIterator#hasNext, but introduced another one in KeyValueBlockIterator#nextBlock, which depends on the behavior of that method. When the first key encountered does not pass the filter, the internal nextBlock field is never intialized. Then a call to nextBlock() results in call to hasNext() which returns true, which recursively calls nextBlock(), again calling hasNext(), etc until the end of the set is reached and an exception is thrown. This skips all valid keys that may occur past the first invalid key.
> Additionally, the current implementation of KeyValueBlockIterator#seekLast depends on the internal RocksDB iterators seekLast() method, which will skip to the last key in the DB regardless of whether it matches the filter or not. This could be different from last key according to the filter.
> This bug was identified while working on HDDS-3869, which adds a strong typing layer before objects are serialized into RocksDB for datanode. Due to RocksDB internals, this changes the database layout so that all prefixed keys are returned at the beginning of the key set, instead of in the end. Since the original layout returned all prefixed keys at the end of the key set, this bug was not evident in any of the original unit tests, since the behavior described above could not occur.
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